Problem: $ { {2} \times \left[\begin{array}{r}{1} \\ {-2} \\ {1}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{2}\times{1} \\ {2}\times{-2} \\ {2}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{2} \\ {-4} \\ {2}\end{array}\right]}$